Aqueous coating compositions having corrosion resistance are generally used as a primer for automobile parts, industrial machines, agricultural machines and the like. When the aqueous coating compositions are applied on a workpiece having poor heat resistance or high heat capacity, the coating compositions have to be dried and cured at a temperature of not more than 80.degree. C. and therefore are generally called as cold curing aqueous coating compositions.
When the cold curing aqueous coating compositions are subjected to spray coating in a coating booth, much amount of paint dust that has not been deposited on a substrate is present. The spray dust is dissolved and dispersed into a booth aqueous solution (that may be called as a collecting solution, hereinafter) in the coating booth to collect. An amount of the collected spray dust in the above way is very large, and if it is disposed by itself, it is waste of paint as well as environmental pollution.
Many efforts to recycle the spray dust collected in the collecting solution have been made, and for example a recycle method is proposed in Japanese Patent Kokai Publication Sho 49 (1974)-51324. The method disclosed in the Japanese Publication comprises collecting the spray dust of the aqueous coating composition with water as a collecting solution to obtain a diluted solution of the aqueous coating composition, filtering the diluted solution through a reverse osmosis membrane or an ultrafiltration membrane, and concentrating and collecting the filtered composition. The concentrated coating composition is then subjected to an adjustment of non-volatile content to a level capable of recycle use. Herein, the "non-volatile" components mean components which are not volatilized away in the aqueous coating composition, including film-forming aqueous resin (such as aqueous alkyd resin and aqueous acrylic resin), pigment and the like.
The method as described in the Kokai Publication, however, has a problem that when filtering and concentrating the diluted solution of the aqueous coating composition with an ultrafiltration membrane, etc., an aqueous solution separated as a filtrate is produced with a very large amount. The collected coating components are reusable, but the aqueous filtrate is generally disposed away. There is a proposal that the aqueous filtrate is returned to the coating booth as the collecting solution and used again for collecting the spray dust.
However, both of the concentrated coating composition and the aqueous filtrate separated from the diluted solution of the aqueous coating composition involve many disadvantages when reusing. For example, since the concentrated coating composition has a reduced volatile content and less amounts of neutralizing amine and solvent, the hydration stability of the resin contained therein decreases, and therefore, an aggregation or separation of the aqueous coating composition may occur. The aqueous filtrate may also aggregate or separate the aqueous coating composition in recycle use.
In order to eliminate the above disadvantages, a method for improving the hydration stability of the resin in recycle use by increasing the hydrophilic property of the resin contained in the original aqueous coating composition is proposed. On applying the above proposal, however, film performances of the coating formed from the aqueous coating composition, such as corrosive resistance, water resistance, and the like, may deteriorate.